The invention relates to A. C. current monitors and more particularly to an electronic device which senses the amount of current in an A. C. load line and causes a fault signal to be developed when the current exceeds a predetermined magnitude.
Many electrical devices such as appliances and machine tools may be damaged or will not operate properly in the presence of high currents. A current monitor may serve to protect such devices by developing a fault signal when the current exceeds a predetermined safe value. In order to serve this function in a reliable, inexpensive and economical manner, such a current monitor should be simple and economical in construction and develop very low voltages during operation.
Current monitors of the prior art do not satisfy these criteria. U.S. Pat. No. 3,916,309 to Faulkes et al discloses a current monitor which is directed primarily to engineering measurement usage rather than protecting electrical devices in use and measures only the short circuit current capacity, not the current in the load line. U.S. Pat. No. 3,934,239 to Mason et al discloses a complex current monitoring device utilizing four windings, two magnetic cores and means for obtaining two regulated D. C. voltages to be applied to operational amplifiers and additional complex circuitry for calibration.
The present invention overcomes these and other disadvantages of the prior art current monitors by providing an A. C. current monitor having solid state components and a single high Q transformer which develops an audible alarm signal when the current in the load line equals or exceeds a predetermined magnitude. The simplicity of design is based on the series connection of the primary winding of a low reactance transformer in the load line. The secondary winding feeds a voltage proportional to the current in the load line to a rectifier and filter circuit which in turn feeds a D. C. voltage proportional to the amplitude of the load line current to the gate of a bidirectional switch such as a triac. The filter circuit contains a potentiometer which may be calibrated to close the switch when the load line current equals or exceeds a predetermined value. When the switch is closed an audible alarm is actuated by an A. C. power source such as the load line being monitored. Optionally, the bidirectional switch may also be connected to a load line circuit breaker such as a second bilateral switch in the load line whose gate is coupled to the first switch through a delay pull-in relay. In addition or as an alternative to an audible alarm, a paging system, visual alarm, remote telephone dialing system or other alarm mechanism may be activated by the fault system.
By using a series primary winding having very low reactance, and solid state components in the sensing circuitry, the device will be very sensitive while a very low voltage drop is maintained in the primary winding and relatively low voltages maintained in the sensing circuitry, thereby protecting sensitive circuit elements and keeping energy dissipation at a low level. For example, with the sensor circuitry developing A. C. voltages of two to three volts and D. C. voltages of zero to four volts the device will monitor currents of seven to twenty amperes. Much larger or smaller load currents may also be monitored using these same principles of the invention. By using a high Q core which is, in combination with the primary and secondary windings, designed to saturate abruptly when the maximum load line current to be monitored is present, the remainder of the circuit is protected against voltage spikes and resulting damage in response to current surges in the primary.
Another feature of the invention is that the rectifier filter circuitry does not provide a pure D. C. signal but a signal having an A. C. component so that the gate is turned on and off intermittently (at the line frequency) when the amplitude of the load line current only marginally exceeds the predetermined current value at which the bilateral switch is designed to close. This opening and closing of the bilateral switch causes a chirping sound which indicates to the listener the degree of overcurrent present in the load line.